1. Field of the Invention
The invention relates to a hydraulic control system of automatic transmission for vehicles and, more particularly, to the hydraulic control system of automatic transmission for vehicles that can perform a skip shift via independent/indirect control and minimize the drive loss of oil pump by means of varying line pressure.
2. Description of the Prior Art
A hydraulic control system of automatic transmission for vehicles controls an output revolution number of planetary gear unit by means of adjusting the operation of clutches and brakes provided in a gear train.
To adjust the operation of the clutches and brakes, the hydraulic control system controls solenoid valves through on/off or duty rate control according to the vehicle speed and load. the control of the solenoid valves forces an oil pressure to flow along a certain line or obstructs the flow of the oil pressure or changes the direction of the flow of the oil pressure.
Accordingly, the oil pressure produced in the oil pump flows along a selected flow way and then applies or releases clutches.
But it is difficult in a the conventional hydraulic control system of automatic transmission for vehicles to perform a skip shift since it is an indirect method of control. as a result, the conventional hydraulic control system has a disadvantage of slow response, i.e, it dosennot respond instantaneously to a driver's intention.
Moreover, since a regulator valve, which regulates the oil pressure to a constant line pressure, adjusts the line pressure only in driving forward and rearward, a line pressure suitable for all driving conditions is used, particularly in driving forward. as a result, more oil pressure than is needed is applied to the clutches at forward third or foruth speed stage and therefore the drive loss of the oil pump is increased. further the load of the oil pump is increased that it has negative affects on fuel consumption ratio.